A PWM converter for use in an electric railway vehicle receives an AC input from a single-phase AC power source between an overhead line and a rail by way of a pantagraph, a transformer and the like, to convert the power from the source into power of a predetermined DC voltage. A capacitor for smoothing the voltage is provided on the DC side of the PWM converter, and an inverter for driving an induction motor is connected to the capacitor. The voltage across the capacitor is detected by a voltage detector, thereby sensing a DC input voltage that is to be applied to the inverter. A current detector is provided on the AC side of the inverter.
A reference output frequency of the inverter is created by adding together, using an adder, a rotation frequency—which is an output of rotation frequency detecting means of the induction motor—and a reference slip frequency—which is an output of slip frequency control. An output current value detected by the current detector is provided to current root-mean-square (RMS) value calculating means, thereby calculating a current RMS value. The current RMS value is transmitted to the adder together with a command current value, thus calculating the reference slip frequency using frequency control means.
A DC input voltage to the inverter is detected by a voltage detector and only its ripple component is derived by a voltage ripple component detecting means. The DC input voltage to the inverter is also input to DC voltage component detecting means and only its DC component is derived therefrom. A divider divides the ripple component by the DC component, to calculate a ripple factor of the DC input voltage, and a multiplier multiples the ripple factor by a reference inverter frequency to calculate an amount of inverter frequency correction. The inverter frequency is calculated by adding the amount of reference inverter frequency to the amount of inverter frequency correction using an adder. This inverter frequency is provided to voltage control means, and a PWM control circuit in turn provides a PWM control signal to the inverter (refer to Patent Document 1, FIG. 1 and its corresponding description).
On the other hand, Non-Patent Document 1 has verified the advantageous effect of Patent Document 1 through experiments. Further, Non-Patent Document 1 (FIG. 7) provides description of a ripple characteristic of a DC power source in a PWM converter for use in a railway vehicle. As for a relationship between the capacitance of DC capacitor and a suppression effect of a beating phenomenon (Non-Patent Document 1 describes based on a beat rate showing how many times a fluctuation range of an inverter output current is greater than that in situations where no beat occurs), Non-Patent Document 1 explains that in order to achieve the suppression effect such that the beat rate is 1.2 times or less, the ripple factor of DC voltage (a ratio of a DC ripple magnitude over a DC average voltage) is reduced to 10% or less. It is described that the capacitance of DC capacitor must be determined to be approximately 30 mF or more (3750 μF per motor) per 8 motors (output rating of approx. 3000 kW), for instance.